CEE (Converged Enhanced Ethernet) is an extended version of Ethernet (Registered Trademark), both of which are standards for communication networks, and the standardization of CEE has now been widely achieved. As the standardization of CEE develops, the integration of LAN (Local Area Network) and SAN (Storage Area Network) has been advancing, and now FCoE (Fiber Channel over Ethernet) technology in which the data flow of FC (Fiber Channel) protocols is sent over the Ethernet network is utilized.
Originally, FC protocols were not developed on the assumption that a packet loss would occur on the communication network. For this reason, when the data of FC protocols is sent over the Ethernet network, it is necessary to achieve a communication in which the loss of communication packets in the network is zero. In the above-mentioned standardization of CEE, such communication is realized by using the congestion control standards referred to as IEEE802.1Qau.
The processes in the IEEE802.1Qau standards are summarized as follows.
Firstly, the length of a queue for each output port of the network device is monitored. Here, the output port at which the length of a queue is larger than or equal to a specified value is determined to be in a congested state, and one data flow is selected from the data traffic flowing through the output port that was determined to be in a congested state. Then, a congestion notification packet is sent to a terminal that is the source of the identified data flow. The source terminal that received the congestion notification packet lowers the transmission rate of the data to be subsequently transmitted, and thereby prevents the continuation of the congested state of the output port.
In IEEE802.1Qau standards, when an output port is congested, it takes a long time to resolve the congestion, as congestion notification is sent only to the source of one data flow arbitrarily selected from the data traffic running through the output port (congested port). It is desirable to give congestion notification to all the sources of data flowing through the congested port. In view of the above, some techniques have been proposed, as below.
In the first technique, a switch itself is provided with hardware that provides the function of monitoring the traffic running through the output ports of the Ethernet switch. In this technique, when an output port is congested, the sources of all the data flow forming the data traffic through the congested port are specified by using the management information that is created by monitoring the data traffic. Then, congestion notification packets are sent to all the specified sources. However, in this technique, the heavy-load processes of monitoring the data traffic and specifying all the data flow through each output port are performed by the switch, and thereby it is necessary to provide the switch with new additional hardware.
In the second technique, every time a data packet arrives at the Ethernet switch, the switch determines whether the output port is congested in order to resolve the port that outputs the packet by using the FDB (Forwarding Database). In this technique, when it is determined that an output port is in a congested state, a congestion notification packet in which the MAC address of the source of a data packet indicated in the arriving data packet is set as the destination address is sent. However, in this technique, even if congestion is detected, a congestion notification packet is sent only to the source of the arriving data packet. Accordingly, a congestion notification packet is not sent, for example, to the sources of other data flows that perform intermittent communication. For this reason, when for example a large number of data packets are transmitted from sources that have not received congestion notification despite the ongoing congestion, the disposal of a large number of data packets or the existence of an extended delay time due to the delay in resolving the congestion is a concern.
Techniques reffered to in the following documents are known.    Document 1:
Japanese Laid-open Patent Publication No. 2000-270002    Document 2:
Japanese Laid-open Patent Publication No. 2002-223223